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The bony gill cover or “operculum” protects the gills from physical damage. It also help fish breath. This is why a fish at rest pulses its jaws and gill covers. With the gill covers are closed and sealed, a fish creates a vacuum to suck in water. Then with a closed mouth, it forces the water over the gills and out the open operculum. In a deformed cutthroat the operculum can’t seal and the gills hang out like red feathers.

Obviously this is a survivable deformity in the Elk River. These fish grow to adult size and appear healthy when caught. But is that OK? What else might be going on that we can’t see?

Image courtesy of Derek Young, Emerging Rivers Guide Services. From a personal fishing trip to the Elk.

First, it’s useful to figure out why gill covers aren’t developing properly in some Elk River cutthroats.

I started by looking for peer-reviewed science on adult gill plate deformity in other wild fish populations. There isn’t much, which can be interpreted as disappointing or as an indication that notable gill plate deformity in wild fish populations is rare. (Which corresponds to anecdotal evidence from anglers who “don’t see this in other rivers” and are generally alarmed when they pull a deformed catch from any lake or river.)

Two papers linked gill plate deformity to pollution:

  • Pulp mill effluent. Sweden. Swedish scientists tracked opercular deformities, what they called “craters” in the cover plate as well as shortened gill covers for eight years. The two years with the highest pulp mill effluent coincided with the peak in deformities. In 1983 and 1984, 20% of perch in the polluted area had shortened gill covers, compared to 1% to 1.4% in unpolluted areas (Lindesjöö et al. 1994.)

“The observed high prevalence of the opercular deformity and the craterous formations were attributed to the effects of the pulp mill effluent.”

  • Domestic wastes, animal husbandry wastes, agricultural pesticides, dioxins, PCBs, and heavy metals. Taiwan. 1994-1997. Scientists collected tilapia from four rivers with various levels of pollution. Gill plate deformities ranged from zero to 14%. Other common deformities were split fins, scale disorientation, gill, lower lip and jaw deformity and opaque cornea. The researchers found correlations between some of the deformities and specific pollutants or water quality parameters that they tested for (dissolved oxygen, ammonia, biological oxygen demand which is a measure of decomposing organic material in the water, suspended solids and the heavy metals copper, chromium, lead, mercury and zinc). For example, split fins were correlated with poor dissolved oxygen and ammonia levels, scale disorientation with suspended solids. But scientists found no specific water quality parameters correlated with operculum deformities (Sun et al. 2009).


“Deformities that occurred at all stations during autumn 1994 included split fins and gill deformities. Other common deformities were scale disorientation, opercular deformities, opaque cornea, and jaw deformities.”

A third study did not discuss the pollution status of the river, but looked at heritability of the deformity:

  • Pollution status of river unknown. Mozambique. Scientists collected four tilapia from a river, all with shortened gill covers. They captive bred them to determine if the deformity was genetic. The next two generations had no deformity, so they concluded that the gill cover deformity was not genetic (Handwerker and Tave 1994).

The take-away from this depauperate collection is that gill deformity in wild fish has been linked to pollution and that the level of pollution in one case has been correlated with frequency of gill plate deformity. The Mozambique study suggests that it may not be genetic.

The limitations of these studies are that none of the fish are salmonids and “pollution” is a pretty broad term. Since the Taiwan study didn’t tease out any correlations, it’s impossible to know what specifically is causing the deformity.

I’m going to dig more into genetics and the specific pollutants in the Elk soon, but first I’m going to look at peer reviewed research in captive bred fish. It turns out that in fish farming, operculum deformity has quite a history.